The present invention relates to calorimetry and, in particular, to calorimetric systems for measuring the power of high energy laser beams.
Although the present invention is concerned primarily with the measurement of high energy laser beam power, its principles will be found useful in lower energy applications or for other comparable types of beams. In general, laser beam power measurements are made both for diagnostic and for operational purposes. For example, an operational shipboard laser system, such as a system used to intercept heat-seeking missiles, operates at a very high energy level and, to assure effectiveness, it is important to be able to determine at any given instant the power or intensity of the output beam. Conventional systems have not proven suitable for such uses. For one reason, they frequently depend on water cooling to protect the various reflectors and heat exchangers and, unfortunately, conventional cooling arrangements are characterized by having an unacceptably slow thermal response time or, in other words, a slow temperature-rise time constant. Further, when the most commonly used systems are scaled upwardly for the higher power levels, they become excessively large and their thermal response time increases with their size and mass.
To avoid such difficulties some arrangements employ a technique in which reflectors are exposed to only a slice of the full beam intensity. Such techniques have the advantage of providing continuous measurement information without disrupting laser beam operation, but their measurements have not been found to be consistently reliable. The problem is that in any cross-sectional area of the beam there may be temporal intensity variations or fluctuations so that any particular slice or sample of this cross-section may not truly represent beam power. A continuously-operable instrument that also is capable of receiving the full beam as opposed to a beam sample, is, as far as is known, an unrealized goal. Most operational systems, consequently, utilize a technique in which the full beam is switched rapidly into the measuring instrument and away from its target. When such a technique is used, there is a need for very rapid thermal responses to minimize the off-time of the beam.
Other difficulties experienced in full-beam measuring devices involve such matters as the ability to avoid the potential danger and feedbacks to the laser and, in particular, the need for insuring reliability and effectiveness of the beam through accurate calibration procedures. As will become apparent, the present arrangement provides a calibrating capability which, coupled to its almost instantaneous thermal response time, permits immediate changes which assure constant effectiveness.